This project investigates primate biobehavioral development through comparative longitudinal studies of rhesus and capuchin monkeys, with special emphasis on characterizing individual patterns of differential behavioral and physiological responses to environmental novelty and challenge and on determining long-term developmental consequences for individuals of different genetic backgrounds reared in different physical and social environments. Research carried out during FY00 (a) characterized a polymorphism in the serotonin 1Db receptor gene (5-HT1Db) for rhesus monkeys, with allele frequencies of p=.82 and q=.18, respecively, in the LCE rhesus monkey colony; (b) demonstrated a predictive relationship between polymorphic status of the 5-HTT gene and measures of behavioral disturbance in 1-month-old rhesus monkey infants; (c) demonstrated significant interactions between 5-HTT polymorphisms and early rearing history for visual orienting capabilities, state control, and activity levels in rhesus monkey neonates; (d) using MRI scans, documented sex differences in total brain volume, but not in total body weight, in yearling rhesus monkeys; (e) characterized developmental changes in circadian rhythmicity of activity patterns in young rhesus monkeys; (f) disclosed a predictive relationship betweencerebrospinal fluid (CSF) concentrations of the primary central serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) and measures of fearfulness and sociality in 1-month-old rhesus monkey infants; (g) characterized significant rearing condition differences in CSF 5-HIAA concentrations that remained stable for rhesus monkeys from infancy to adolescence; (h) documented rearing condition differences in CSF concentrations of corticotrophin releasing hormone (CRH) following a mild environmental challenge in rhesus monkey juveniles; (i) demonstrated that the CRH type 1 receptor antagonist analarmin suppresses hypothalamic-pituitary-adrenal (HPA) activity and behavioral manifestations of fearfulness in adult male rhesus monkeys; and (j) documented predictive relationships between HPA reactivity and a variety of specific measures of social behavior and environmental exploration in capuchin monkey juveniles that parallelled previous findings in rhesus monkeys, despite major differences between the two species in overall rates of social activity and in social group structure and dynamics.